P-035 - First report of a receptor-like kinase with an intracellular LRR domain in plant immunity

Plant immunity is triggered externally by recognizing pathogen-associated patterns through receptor-like proteins (RLPs) or receptor-like kinases (RLKs). Some RLKs act as co-receptor in the initial recognition phase and are essential for regulating downstream signaling. In tomato, Cf-4 is an RLP that recognizes the effector Avr4 of Fulvia fulva (formerly known as Cladosporium fulvum) and forms a complex with the RLKs SOBIR1 and BAK1 to initiate downstream signal transduction. However, the downstream signaling of this complex remains not fully understood. In the present study, the downstream signaling of Cf-4 resistance was investigated by TurboID mediated proximity-dependent labelling. Consequently, immune-related proteins, including a novel RLK belonging to the BIR protein family, were significantly enriched upon Avr4 perception. In contrast to typical RLKs containing only an extracellular leucine-rich repeat (LRR) domain, in this study, we present the first report of a tomato RLK that has both extra- and intracellular LRR domains. Notably, the same BIR protein structure is also present in Nicotiana benthamiana but not in Arabidopsis. Unlike the previously characterized Arabidopsis BIR proteins, it appears to positively regulate immunity, while interacting more strongly with SOBIR1 than with BAK1. This atypical RLK protein plays a critical role in immune signaling, opening new avenues for understanding co-receptor evolution and their function in plant immunity.